B10 The human nervous system

  • Created by: msahay
  • Created on: 24-08-17 10:18

B10.1 Principles of homeostasis


  • Regulation of the internal conditions of a cell/organism to maintain optimum conditions for function, in response to internal and external changes.
  • Important for maintaining optimal conditions for enzyme action and cell functions
  • Internal conditions controlled - body temperature, water content of body, blood glucose concentration. 
  • Detecting changes and responding to them involves automatic control systems - nervous and chemical. 

Demands of a control system

  • Receptors - cells that detect stimuli (changes) - part of nervous or hormonal
  • Co-ordination centres - receive and process info from receptors. Send signals - includes brain, spinal cord.
  • Effectors - muscles or glands that bring about the actual responses to the stimulus that has been received. Restore conditions in body to optimum levels.
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B10.2 Structure and function of nervous system

The nervous system

  • Receptors = cells that detect changes in the environment (internal and external)
  • Receptor cells = have nucleus, cytoplasm, cell membrane found in sense organs such as skin and eyes. 

How the nervous system works

1) Sensory receptor detects stimuli

2) Sensory receptor sends info as impulse, passing along a neurone

3) Impulse travels along sensory neurone till it reaches CNS - central nervous system. 

4) Brain co-ordinates response and sends out impulses along motor neurones 

5) Impulse reaches effector - muscles that contract or glands that secrete chemical substances.

Stimulus ----> Receptor ----> Sensory neurone -----> CNS ----->Motor neurone -----> Effector

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B10.2 Structure and function of nervous system 2


Cells called receptors detect stimuli (changes in the environment)

Impulses from receptors pass along sensory neurones to the brain or spinal cord (CNS)

The brain co-ordinates the response

Impulses from brain pass along the motor neurones to the effector organs

Effector organs - muscles contract or gland secretes chemicals.

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B10.3 Reflex actions

What are reflexes?

  • Reflex actions are automatic, rapid and do not involve the concious part of the brain
  • Reflexes help avoid danger and take care of basic body functions e.g. breathing

Reflex arc 

  • Receptor sends impulse along sensory neurone to the CNS
  • Impluse from CNS passes along a relay neurone in spinal cord 
  • Impulse from spinal cord passes along a motor neurone
  • Impulse arrives at effector organ (muscle or organ)

Key point: impulse bypasses the concious part of brain - making it very fast


Synpases are junctions between neurones form physical gaps. Impulses have to cross these by diffusing as chemicals. Slower than the electrical impulse in the neurones but allows it to cross.

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B10.3 Reflex actions 2

Touching a hot object

1) Hot object touched, receptor in skin is stimulated. An electrical impulse from a receptor passes along a sensory neurone to the CNS - spinal cord

2) Impulse from sensory neurone arrives at the synapse with a relay neurone. Chemical released and diffuses across synapse. New impulse travels along the relay neurone

3) Impulse from relay neurone arrives at the synapse with a motor neurone. Chemical released and diffuses across synapse. New impulse travels along the motor neurone

4) Impulse reaches effector organ - stimulated to respond. Muscle contrcts moving hand away from hot object. 

In reflex action, co-ordinator is the relay neurone in spinal cord/unconcious part of brain - making it so fast

Stimulus ---> Receptor ---> Sensory neurone ---> Synapse ---> Relay neurone ---> Synapse ---> Motor neurone ---> Effector ---> Response 

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B10.4 The brain

Structure of the brain

  • Brain protected by skull and protective membrane called meniges. 
  • Cerebral cortex - concerned with conciousness, intelligence, memory and language
  • Cerebellum - concerned with co-ordinating muscular activity and balance
  • Medulla - concerned with unconcious activities e.g. breathing, heartbeat, movements of the gut
  • Hypothalamus - controls body temperature
  • Pituitary gland - produces hormones 

Finding out about the brain

  • Studying patients with brain damage - matching changes in behaviour or memory with the damaged area has helped to map out the functions of the brain
  • Electrically stimulating different parts of the brain - people experience behaviour when parts of brain are stimulated
  • MRI scans 
  • Brain = complex and delicate. Difficult to treat mental problems because drugs don't always reach brain and surgery can cause unintended damage.
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B10.5 The eye

Functions of the structures in the eye

  • Sclera - white outer layer of eye, tough and strong 
  • Cornea - front of eyeball, lets light into eye, curved surface focuses light onto retina
  • Iris - muscle that controls size of pupil and amount of light hitting retina. Dim light = enlarged pupil, Bright light = smaller sized pupil 
  • Lens - focusing light to produce a clear image on retina, jelly to maintain shape in eyeball
  • Suspensory ligaments/ciliary muscles - change the shape of lens to fine focus light onto retina

How we see 

1) Light hits retina

2) Light sensitive cells stimulated 

3) Impulses sent to brain along sensory neurones in the optic nerve.

4) Brain receives messages and interprets them as visual image

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B10.5 The eye 2

Focusing the light 

Cornea and jelly change the direction of the light rays onto the retina - same amount refracted

Ciliary muscles - changes the shape of the lens of the eye - changes amount of lens refracting light 

Basic principle:


Light passes through the cornea and lens which focus it on the retina 

Image upside down on the retina 

Messages sent to brain

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B10.6 Common problems of the eye

Accomodation = process of changing shape of lens of eye to focus on near/distant objects

Distant object - ciliary muscles relax, suspensory ligaments pulled tight, lens is pulled flat. Refracts light rays slightly 

Near object - ciliary muscles contract, suspensory ligaments loosen, lens is thicker and curved. Refracts light rays strongly 

Myopia - short sightedness, can see close up things, not far off

  • Light focused in front of retina, not actually hitting it. 
  • Lens may be too curved or long
  • Wearing concave lens - spreads out light from distant objects before reaching eye, thicker lens

Hyperopia far sightedness, can see fa2r off things, not close up 

  • Lens - too flat or thin, short eyeball 
  • Light focused behind retina
  • Wearing convex lens - bring light rays together before they reach the eye, thinner lens 
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B10.6 Common problems of the eye 2

The role of new tech

  • Contact lenses 

Placed on surface of eye

Hard contact lenses made of hard material and have to be removed and kept sterile overnight

Soft contact lenses made of flexible material - can be disposable, or for a month can be worn

  • Laser eye surgery 

Lasers used to treat myopia reduce thickness of cornea 

Lasers used to treat hyperopia change curve of the cornea 

  • Replacement lenses 

Adding another lens in the eye itself. Permanent contact implanted into eye or faulty lens replaced by atrificial lens - latter includes risk of infection and damage to cornea

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Very useful in revision, thanks a lot.

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